I'm not sure, just speculating on what they've done before: R6 -> R6 Pro, R3 -> R3 Pro. The R5 is relatively new so hasn't got the Pro treatment yet, but I'm sure it's coming.
Hard to say what they'll change, but maybe put in a beefier and even quieter amp stage, maybe a tube option (doubtful), maybe a switch to the new Sabre flagship DACs. Who knows.
Of course with Covid things are still up in the air, so if they have already started on a Pro design, we may only see it towards the end of next year or into 2022. Or they may decide to skip the 8 Pro and bring in a 7-series dap to bridge the gaping hole between the R6 Pro and R8 price points.
Time will tell. My point is something new is always coming (especially from HiBy), and I'm not in a rush. That said, in a year's time if used R8 prices hover around $1k, it would be a tasty update on the R6 Pro for sure.
@twister6 I really hope you'll be contacting the R8 to the R6 Pro in your review so we know what the extra $1300 in today's market buys you.